1. Field of the Invention
This invention relates to hydraulic fluid compositions for use in hydraulic systems of automobiles, etc. More particularly, it relates to water-insensitive and stable brake fluid compositions which can maintain high boiling points even when water is added to the initial fluid compositions.
2. Description of the Prior Art
A great number of hydraulic fluid compositions have been suggested in the art. Commonly, the hydraulic fluids are made up of three principal units. The first is a base or lubricant for the system which may include heavy bodied fluids such as polyglycols. Diluents such as glycol ethers and alcohols, which are employed for the purpose of controlling the viscosity of the fluid, form the second basic unit. Finally, the third unit is represented by an inhibitor system comprising small quantities of inhibitors which are added, for example, to reduce oxidation and to minimize corrosion. Although the hydraulic fluids of the prior art possesses one or more of these desired characteristics. they all suffer from one or more disadvantages. Fluids known in the art are hygroscopic and water-sensitive. Such properties are undesirable under many practical conditions of use. In particular, absorbed water tends to increase the vapor pressure of the fluid composition and to lower its boiling point, both of which are serious disadvantages when the fluid composition is to be used under conditions of high temperature, because they contribute to causing vapor lock.
Some attempts have been made to develop an improved hydraulic fluid which does not cause vapor lock (in other words, not sensitive to moisture, or not subject to decreases in boiling point). British Pat. No. 1,214,171, discloses an improved hydraulic fluid which comprises a borate ester from 54.5to 92% by weight, and a polyoxyalkylene glycol mono or diether from 3 to 43% by weight, based on the total weight of the fluid composition. Such an hydraulic fluid has demonstrated quite small decreases in boiling point due to water absorption (wet equilibrium reflux boiling point) such that it is capable of meeting the requirements for grade DOT 4 with respect to the wet equilibrium reflux boiling point, when the hydraulic fluid is tested according to the hydraulic fluid specifications of the U.S. Department of Transportation [DOT]. The hydraulic fluid in the above British Patent liberates a precipitate under test conditions, wherein the fluid is allowed to stand at room temperature following heating at 100.degree.C for 120 hours. The precipitation may be caused by hydrolysis of the borate ester (one of the basic components of the fluid) with the absorbed water under the test conditions (e.g. high temperature) to form boric acid which is easy to precipitate. The precipitation must be avoided because it results in the formation of deposits in brake devices, with consequent lowering of brake efficiency.
A need exists therefore, for a hydraulic fluid which is capable of fully meeting the standards of DOT 4 in both the wet equilibrium reflux boiling point test, and in the evaporation test.